. Fragile X syndrome is among the most common human single gene disorders and the leading cause of inherited mental retardation. The most common mutation reduces expression of the FMR1 gene through expansion and methylation of an unstable CGG trinucleotide repeat. Two similar repeats are found nearby on the X chromosome which can expand and become unstable. All three sequences lead to cytogenetically visible fragile sites in their largest forms. Two sites, FRAXA and FRAXE lead to mental retardation associated with the FMR1 and FMR2 genes, respectively. The third, FRAXF, appears to be benign. This project seeks to continue study of these two genes leading to mental retardation. It will continue analysis of functional aspects of FMR1 and proposes to extend these studies to FMR2. It plans to study the roles of FMR1-related genes FXR1 and FXR2. These genes are similar in sequence to the FMR1 gene, and appear to interact with the FMR1 gene product in vivo. All three genes have properties of RNA binding proteins and may be involved in regulating RNA metabolism. The RNA targets of these three proteins may be important to the finding of mental retardation. FMR2 is unrelated in sequence and has features reminiscent of a transcription factor; however, it is highly expressed in portions of the brain associated with learning and memory, explaining the mental retardation seen in individuals lacking the protein. Specific aims are: 1) identification and characterization of mRNAs mis-regulated by absence of or lesions in FMR1 and FXR2; 2) development of mouse models to delineate developmental and temporal requirements for FMR1 in learning and behavior, and to study structure/ function aspects of FMR1.; 3) continued characterization of FMR1 and interacting proteins through two hybrid and mutation screens, 4) targeted disruption of the murine Fxr2 locus and description of the phenotype. If viable homozygotes can be produced, this animal will be crossed with the Fmr1 knockout mouse to determine the effect of the double knockout; 5) characterization of the FMR2 gene defective in FRAXE mental retardation.